Resistive switching memory

ABSTRACT

In one embodiment of the present invention, a memory cell includes a first resistive switching element having a first terminal and a second terminal, and a second resistive switching element having a first terminal and a second terminal. The memory further includes a three terminal transistor, which has a first terminal, a second terminal, and a third terminal. The first terminal of the three terminal transistor is coupled to the first terminal of the first resistive switching element. The second terminal of the three terminal transistor is coupled to the first terminal of the second resistive switching element. The third terminal of the three terminal transistor is coupled to a word line.

TECHNICAL FIELD

The present invention relates generally to memory devices, and moreparticularly to resistive switching memory.

BACKGROUND

Semiconductor industry relies on device scaling to deliver improvedperformance at lower costs. Flash memory is the mainstream non-volatilememory in today's market. However, Flash memory has a number oflimitations that is posing a significant threat to continued advancementof memory technology. Therefore, the industry is exploring alternativememories to replace Flash memory. Contenders for future memorytechnology include magnetic storage random access memory (MRAM),ferroelectric RAM (FeRAM), and resistive switching memories such asphase change RAM (PCRAM), metal oxide based memories, and programmablemetallization cell (PMC) or ionic memories. These memories are alsocalled as emerging memories.

To be viable, the emerging memory has to be better than Flash memory inmore than one of technology metrics such as scalability, performance,energy efficiency, On/Off ratio, operational temperature, CMOScompatibility, and reliability.

One of the challenges of memory design relates to cell margin. Forexample, the switching element, which switches between a first state anda second state has to maintain a sufficient difference between the twostates so that a subsequent read operation may distinguish between them.If the difference between the two states becomes smaller than thesensitivity of the reading process, then the memory cell may lose thestored data.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the present invention, a memory cellcomprises a first resistive switching element having a first terminaland a second terminal, a second resistive switching element having afirst terminal and a second terminal, and a three terminal transistor.The three terminal transistor has a first terminal, a second terminal,and a third terminal. The first terminal of the three terminaltransistor is coupled to the first terminal of the first resistiveswitching element. The second terminal of the three terminal transistoris coupled to the first terminal of the second resistive switchingelement. The third terminal of the three terminal transistor is coupledto a word line.

In accordance with an alternative embodiment of the present invention, amemory cell comprises a first resistive switching element having acathode terminal and an anode terminal, and a second resistive switchingelement having a cathode terminal and an anode terminal. The memory cellfurther comprises a bipolar transistor having a first emitter/collector,a second emitter/collector, and a base. The first emitter/collector iscoupled to the cathode terminal of the first resistive switchingelement. The second emitter/collector is coupled to the cathode terminalof the second resistive switching element. The base is coupled to a wordline. The anode terminal of the first resistive switching element iscoupled to a first bit line of a bit line pair. The anode terminal ofthe second resistive switching element is coupled to a second bit lineof the bit line pair. The memory cell is configured to store a first ora second memory state.

In accordance with an alternative embodiment of the present invention, amemory cell comprises a first resistive switching element having a firstterminal and a second terminal, and a second resistive switching elementhaving a first terminal and a second terminal. The memory cell furthercomprises a transistor having a first source/drain and a secondsource/drain. The first source/drain is coupled to the first terminal ofthe first resistive switching element, and the second source/drain iscoupled to the first terminal of the second resistive switching element.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 illustrates a schematic circuit of a differential memory cellarray in accordance with an embodiment of the present invention;

FIG. 2, which includes FIGS. 2A-2D, illustrates a structuralimplementation of a differential memory cell array in accordance with anembodiment of the present invention, wherein FIG. 2A illustrates a topview while FIGS. 2B and 2C illustrate cross-sectional views and FIG. 2Dillustrates a magnified view of a resistive switching element in oneembodiment;

FIG. 3, which includes FIGS. 3A-3E, illustrates the operation of thememory cell array in accordance with embodiments of the presentinvention, wherein FIG. 3A illustrates activating a memory cell, whereinFIG. 3B illustrates a memory cell of the memory cell array during awrite operation in accordance with an embodiment of the presentinvention, wherein FIG. 3C illustrates the operation of the memory cellincluding parasitic effects in accordance with an embodiment of thepresent invention, wherein FIG. 3D illustrates the operational states ofthe memory cell in accordance with embodiments of the present invention,and wherein FIG. 3E illustrates a memory cell of the memory cell arrayduring a read operation in accordance with an embodiment of the presentinvention;

FIG. 4, which includes FIGS. 4A-4E, illustrates a memory cell array inaccordance with an alternative embodiment of the present invention,wherein FIGS. 4B-4E illustrate a structural implementation of the memorycell illustrated in FIG. 4A in accordance with embodiments of thepresent invention, wherein FIG. 4B illustrates a top view, wherein FIGS.4C and 4D illustrate cross-sectional views while FIG. 4E illustrates amagnified cross-sectional view of the resistive switching element;

FIG. 5 illustrates a further embodiment of the differential memory cellarray having a NFET and having blocking diodes in accordance with anembodiment of the present invention;

FIG. 6 illustrates a further embodiment of the differential memory cellarray having a PFET and blocking diodes in accordance with an embodimentof the present invention;

FIG. 7, which includes FIGS. 7A-7C, illustrates a memory deviceimplementing embodiments of the invention; and

FIG. 8 illustrates a schematic block diagram of a system implementingembodiments of the invention.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of various embodiments are discussed in detailbelow. It should be appreciated, however, that the present inventionprovides many applicable inventive concepts that can be embodied in awide variety of contexts. The embodiments discussed are merelyillustrative of a few ways to make and use the invention, and do notlimit the scope of the invention.

The present invention will be described with respect to variousembodiments in a specific context, namely ionic memories such asprogrammable metallization cells (also called as conductive bridgingmemories, nanobridge memories, or electrolytic memories). The inventionmay also be applied, however, to other types of memories, particularly,to any resistive memory such as two terminal resistive memories.

Embodiments of the present invention describe a 1T-2R (onetransistor-two resistors) differential memory cell. As a consequence,embodiments of the invention improve cell margin without significantlyincreasing cell size. For example, the resistance states of the tworesistors of the memory cell may drift significantly (at least more thanwhat is allowed for a 1T-1R memory cell) without losing data.Advantageously, in the memory cell design described below in variousembodiments, both resistive elements may have to completely lose datafor the memory cell to fail data retention.

In various embodiments, advantageously, both the resistive memoryelements of the 1T-2R memory cell may be switched simultaneously in asingle program/erase operation. In other words, in various embodimentsdescribed below, writing data to one side of cell automatically writesopposite data to other side of cell. This avoids a significant increasein switching time relative to a 1T-1R memory cell.

Embodiments of the invention describe a single transistor differentialmemory cell, for example, using a single lateral bipolar junctiontransistor access device or a field effect transistor access device insome embodiments. As a consequence, the cell size may be reducedcompared to a conventional two transistor differential memory celldesign with two transistors. For example, a memory cell having 8F² cellarea may be possible using a lateral bipolar transistor described inFIGS. 1-4, where F is the technology feature size (pattern line and/orspace), e.g., F may be 50 nm. Alternatively, a cell size less than about12F² may be formed using field effect transistors as described in FIGS.5-6.

FIG. 1 illustrates a schematic circuit of a differential memory cellarray in accordance with an embodiment of the present invention.

Referring to FIG. 1, a memory cell array comprises an array of memorycells, for example, a first memory cell (CELL1), a second memory cell(CELL2), a third memory cell (CELL3), and a fourth memory cell (CELL4).Each of the memory cells in the memory array may be a one transistor-tworesistors (1T-2R) memory cell in one or more embodiments.

The memory cell array may be programmed, erased, read using a pluralityof voltage rails. The memory cell array may be connected through aplurality of word lines WL, a plurality of bit lines BL. Each cell isoperated using a pair of bit lines, for example, a first bit line pairBL1 and BL1# for the first memory cell and the fourth memory cell, thesecond bit line pair BL2 and BL2# for the second memory cell and thethird memory cell. The memory array includes a plurality of word lines,for example, a first word line WL1 serving the first memory cell and thesecond memory cell, and the second word line WL2 serving the thirdmemory cell and the fourth memory cell.

Each memory cell comprises a first resistive switching element 10, asecond resistive switching element 20, and a bipolar transistor 50. Thefirst resistive switching element 10 has a cathode 11 and an anode 12.Similarly, the second resistive switching element 20 has a cathode 21and an anode 22. In one or more embodiments, the first resistiveswitching element 10 and the second resistive switching element 20comprise the same type of switching element. In one or more embodiments,the first resistive switching element 10 and the second resistiveswitching element 20 have similar characteristics, e.g., similarresistive states in OFF and ON states, and similar threshold voltages.Alternatively, the first resistive switching element 10 and the secondresistive switching element 20 may be asymmetric, e.g., the firstresistive switching element 10 may have a lower off state resistancethan the second resistive switching element 20 or the first resistiveswitching element 10 may have a lower switching threshold than thesecond resistive switching element 20. Switching threshold is voltage atwhich the resistive switching element changes resistance from either thehigh resistive state to the low resistive state (program threshold, Vtp)or from the low resistance state to the high resistance state (erasethreshold Vte). The program and erase thresholds are not necessarily thesame and may be a function of the time the programming or erasingvoltage is applied.

In various embodiments, the first and the second resistive switchingmemory elements 10 and 20 may comprise resistive switching memories thatswitch based on thermal, electrical, and/or electromagnetic effects. Thefirst resistive switching element 10 and the second resistive switchingelement 20 are configured to be programmed (high to low resistancestate) by the application of a positive voltage at the anode relative tothe cathode and erased (low to high resistance state) by the applicationof a negative voltage at the anode relative to the cathode.

As illustrated, the anode 12 of the first resistive switching element 10is coupled to one of the first bit line pair BL1 while the anode 22 ofthe second resistive switching element 20 is coupled to the other of thefirst bit line pair BL1#. The cathode 11 of the first resistiveswitching element 10 is coupled to a first emitter/collector region 121of the bipolar transistor 50. Similarly, the cathode 21 of the secondresistive switching element 20 is coupled to a second emitter/collectorregion 122 of the bipolar transistor 50. The base of the bipolartransistor 50 is coupled to a word line, for example the base of thebipolar transistor 50 of the first memory cell is coupled to the firstword line WL1.

Thus, as will be described further in FIG. 3, the memory cell is adifferential cell using a difference in the resistance between the firstresistive switching element 10 and the second resistive switchingelement 20 to store a memory state. For example, a first memory state ofthe differential memory cell may correspond to a low resistance state atthe first resistive switching element 10 and a high resistance state atthe second resistive switching element 20. A second memory state of thedifferential memory cell may correspond to a high resistance state atthe first resistive switching element 10 and a low resistance state atthe second resistive switching element 20.

FIG. 2, which includes FIGS. 2A-2D, illustrates a structuralimplementation of a differential memory cell array in accordance with anembodiment of the present invention. FIG. 2A illustrates a top viewwhile FIGS. 2B and 2C illustrate cross-sectional views and FIG. 2Dillustrates a magnified view of a resistive switching element in oneembodiment. FIG. 2 may be an implementation of the memory cell circuitillustrated in FIG. 1.

Referring to FIG. 2A, a plurality of word lines, e.g., a first word lineWL1, a second word line WL2, a third word line WL3, and a fourth wordline WL4 are disposed within and/or over a substrate 100. In variousembodiments, the plurality of word lines may be doped semiconductorembedded within and/or the substrate 100 (FIG. 2C). The plurality ofword lines may be isolated from each other by isolation regions, forexample, trench isolation 160.

As illustrated in the top of view of FIG. 2A, a plurality of bit linesare formed over the substrate 100. Referring to FIG. 2B, a well region110 is formed over a substrate 100. The well region 110 may have a firstdoping type (n-type or p-type), which is opposite to the doping type ofthe substrate 100 (noting that FIG. 2 illustrates a case in which thefirst doping type is n-type). Therefore, the well region 110 may performthe function of the plurality of word lines. Thus, the plurality of wordlines is formed as a buried diffusion region in the substrate 100. Asillustrated in FIG. 2C, the trench isolation 26 is deeper than the depthof the well region 110 and therefore forms a plurality of word lines(WL1, WL2, WL3, WL4, and so on).

A plurality of doped regions 120 is formed on a top surface of the wellregion 110. The plurality of doped regions 120 has the second dopingtype, which is the reverse of the doping of the well region 110.Consequently, a bipolar transistor 50 (FIG. 1, 2B) is formed between theplurality of doped regions 120 and the well region 110. As illustratedin FIG. 2B, a first emitter/collector region 121 of the plurality ofdoped regions 120, a base region 123 of the well region 110, and asecond emitter/collector region 122 form a lateral bipolar transistor50. In various embodiments, illustrated in FIG. 2, the first doping typeis n-type and the second doping type is p-type. The base region 123 ofthe bipolar transistor is part of the well region 110, and therefore thebase of the bipolar transistor 50 is directly electrically coupled tothe word line forming the well region 110.

Additionally, parasitic transistors are formed with this structure. Forexample, the first emitter/collector region 121, a portion of the wellregion 110, and the substrate 100 form a first parasitic bipolartransistor 51 while the second emitter/collector region 122, anotherportion of the well region 110, and the substrate 100 form a secondparasitic bipolar transistor 52.

The plurality of doped regions 120, which includes the firstemitter/collector region 121 and the second emitter/collector region122, are coupled through a plurality of contacts 130. The plurality ofresistive switching elements 140 within the metallization layers isdisposed over the plurality of contacts 130. The plurality of resistiveswitching elements 140 may be disposed in any metallization levels overthe substrate 100. The plurality of resistive switching elements 140 maybe formed over one or more metallization levels in various embodiments.

In one or more embodiments, each of the plurality of resistive switchingelements 140 may comprise an ionic memory. Such ionic memory may involvecells based on anion migration or cation migration. An example of anionic memory includes a conductive bridging random access memory. TheCBRAM may comprise a solid electrolyte layer sandwiched between an inertelectrode and an electro-chemically active electrode. In otherembodiments, each of the plurality of resistive switching elements 140may comprise metal oxide memory, which switches based on electroniceffects, e.g., based on resistive switching metal oxides. In analternate embodiment, each of the plurality of resistive switchingelements 140 may switch based on thermal effects and may comprise aphase change memory unit in alternative embodiments.

In various embodiments, as will be described using FIG. 3, each of theplurality of resistive switching elements 140 may comprise any type ofmemory, which requires lower erase currents than programming currents.

The plurality of resistive switching elements 140 includes a firstresistive switching element 10 and a second resistive switching element20 (as also illustrated in FIG. 1). For ease of understanding, the anode12 of the first resistive switching element 10 and the cathode 11 of thefirst resistive switching element 10, and the anode 22 of the secondresistive switching element 10 and the cathode 21 of the secondresistive switching element 20 are also illustrated in FIG. 2B.

Referring to FIG. 2B, the plurality of bit line contacts 150 is disposedover the plurality of resistive switching elements 140. The plurality ofbit line contacts 150 may couple to the bit lines disposed over theplurality of resistive switching elements 140. In one or moreembodiments, the plurality of bit line contacts 150 may comprise viascontacting the top electrode layer of the resistive switching element140 and a metal line forming a bit line.

FIG. 2D illustrates a magnified view of a resistive switching element inaccordance with an embodiment of the present invention.

In one embodiment, each of the plurality of resistive switching elements140 (e.g., a first resistive switching element 10) comprises anelectrolytic memory. In various embodiments, each of the plurality ofresistive switching elements 140 such as the first resistive switchingelement 10 may comprises a cathode electrode layer 15, a switching layer16, and an anode electrode layer 17. The cathode electrode layer 15 maybe an inert or active (participating in the switching mechanism)electrode, which forms the cathode 11, and may be coupled to theplurality of contacts 130 (see FIG. 2B). The cathode electrode layer 15may be formed as a bottom electrode of the resistive switching elementin various embodiments.

In one or more embodiments, the switching layer 16 may comprise a layerthat is capable of changing conduction from relatively low conductanceto relatively high conductance. In various embodiments, the layer(switching layer 16) may comprise a chalcogenide material such as agermanium based chalcogenide, e.g., a copper doped GeS₂ layer. In analternative embodiment, the layer may comprise silver doped GeS₂. Inother embodiments, the layer may comprise copper doped WO₃, Cu/Cu₂S,Cu/Ta₂O₅, Cu/SiO₂, Ag/Zn_(x)Cd_(1-x)S, Cu/Zn_(x)Cd_(1-x)S,Zn/Zn_(x)Cd_(1-x)S, GeTe, GST, As—S, Zn_(x)Cd_(1-x)S, TiO₂, ZrO₂, SiO₂.In some embodiments, the layer may comprise a plurality of layers andmay include bilayers such as Ge_(x)Se_(y)/SiO_(x), Ge_(x)Se_(y)/Ta₂O₅,Cu_(x)S/Cu_(x)O, Cu_(x)S/SiO₂ and combinations thereof.

In one embodiment, the switching layer 16 may comprise transition metaloxides which may change conductivity due to the formation of chargedpoint defects such as oxygen vacancies, other charge complexes, or othermechanisms so as to increase or decrease conductivity in the layer. Theswitching layer 16 may comprise metal oxides such as hafnium oxide,gadolinium oxide, and other such materials doped with copper, silver, Teor other transition metals in various embodiments. In other examples, ametal oxide based switching layer 16 may comprise NiO_(x), TiO_(x),Al₂O₃, Ta₂O₅, CuO_(x), WO_(x), CoO, Gd₂O₃, TiO_(x), FeO_(x), chromiumdoped perovskite oxides such as SrZrO₃, (Ba, Sr)TiO₃, SrTiO₃, copperdoped MoO_(x), copper doped Al₂O₃, copper doped ZrO₂, Al doped ZnO,Pr_(0.7)Ca_(0.3)MnO₃, as examples.

The anode electrode layer 17, which forms the anode 12, may comprise anelectrochemically active metal such as silver, copper, zinc, Ti, Ta,alloys or layered structures of Cu and Te, alloys or layered structuresof Ti and Te, alloys or layered structures of Ta and Te, and others invarious embodiments. The anode electrode layer 17 may also have a caplayer such as titanium nitride or tantalum nitride (as well as othersuitable materials) in various embodiments.

FIG. 3, which includes FIGS. 3A-3E, illustrates the operation of thememory cell array in accordance with embodiments of the presentinvention.

In various embodiments, as also illustrated in FIG. 2, a lateral bipolartransistor is used to implement blocking diodes and as an access devicefor the memory cell. Advantageously, such an implementation simplifiesthe interconnect structure for the memory cell array. As described inthe above embodiment, the base of the bipolar transistor is directlyelectrically coupled to the word line, which is formed as a burieddiffusion region.

In accordance with an embodiment of the present invention, the firstmemory cell may be activated by applying a positive write voltage−V_(WRITE) at the first bit line BL1 and a negative write voltage−V_(WRITE) at the other first bit line BL1#. The first word line WL1 ispulled up to one intermediate voltage, for example, to 0V. The secondmemory cell and the third memory cell are inhibited because of theabsence of potential difference between the bit line pairs. The secondword line WL2 (as well as the remaining word lines in the array) may bepulled to high (positive write voltage +V_(WRITE)) to turn off thecorresponding bipolar transistor. Thus, the fourth memory cell is alsoinhibited by applying a positive voltage on the base of the bipolartransistor to turn off the bipolar transistor.

FIG. 3B illustrates a memory cell of the memory cell array during awrite operation in accordance with an embodiment of the presentinvention. The illustration uses an example for programming R1 anderasing R2. It will be understood by those with ordinary skill in theart that reversing the respective BL voltages will erase R1 and programR2.

As described above, during a write operation, a positive write voltage+V_(WRITE) is applied at the first bit line BL1 and a negative writevoltage −V_(WRITE) (−Vwrite may have different absolute magnitude than+Vwrite) is applied at the other first bit line BL1#, and anintermediate voltage (0V) is applied at the first word line WL1. As aconsequence, a program current IPR flows through the first resistiveswitching element 10 and an erase current IER flows through the secondresistive switching element 20. Thus, while the first resistiveswitching element 10 transitions from a high resistance state to a lowresistance state, simultaneously, the second resistive switching element20 transitions from a low resistance state to a high resistance state.

Before the switching of the states of the first and the second resistiveswitching elements 10 and 20, the bit line voltages are asserted. Thefirst emitter/collector region 121 of the bipolar transistor 50 isinitially at about 0V and the base current (I_(b)) is negligible(leakage current). The write voltage of the bit line may be coupledthrough a high resistance state R1 of the first resistive switchingelement 10. Thus, the voltage at the anode 12 of the first resistiveswitching element 10 (Vbl) increases after the application of the bitline voltage.

For better understanding of the operation of the memory cell, thecurrent flowing through each of the device is described below. However,these equations include many assumptions regarding the operation anddevice physics. As is known to one skilled in the art, more accurateresults may be obtained by detailed modeling combined with experimentaldata and/or direct experimental measurements. Therefore, the followingequations are provided for intuitive understanding.

During programming of the first resistive switching element 10, thevoltage at the anode 12 is given as

${Vbl} = {{( \frac{R\; 1}{{R\; 1} + {Rl}} )*( {{+ {Vwrite}} - {Vbe}} )} + {{Vbe}.}}$

Vbe is the potential at the first emitter collector region 121 and isabout the same as the potential V1 at the cathode 11 of the firstresistive switching element 10. Rl is the series resistance between+Vwrite and Vbl resulting from both the program/erase control circuitsand memory array access circuits. Rl may be a small number with respectto R1.

The emitter current Ie through the first emitter/collector region 121,which is the same as the program voltage IPR, may be written as

${I\; e} = {\frac{{Vwrite} - {Vbe}}{{R\; 1} + {Rl}} = {{Ipr}.}}$

Thus the collector current Ic may be determined as

${Ic} = {( \frac{\beta}{\beta + 1} )*{( \frac{{Vwrite} - {Vbe}}{{R\; 1} + {Rl}} ).}}$

Here, β is the common emitter current gain given by the ratio of thecollector current Ic to the base current Ib. Thus, a portion of theemitter current Ie flows into the first word line WL1 as base currentIb, and a remaining portion Ic flows into the second resistive switchingelement 20.

Thus, immediately after (during) programming the first resistiveswitching element 10 the second resistive switching element 20 is eraseddue to the flow of the collector current Ic. However, less current flowsduring the erase process of the second resistive switching element 20than in the program process of the first resistive switching element 10.

However, advantageously, many memory systems based on metal oxidememories and programmable metallization cell memories such as CBRAMrequire less erase current than program current. Therefore, embodimentsof the invention may be applied to such memories.

The maximum erase current in the second resistive switching element 20,which is the collector current of the bipolar transistor 50 may bedetermined as

$\frac{{V\; 2} + {Vwrite}}{R\; 2} = {{Ic} = {{{Max}.{Erase}}\mspace{14mu}{{Current}.}}}$

Here, R2 is the low resistance state of the second resistive switchingelement 20. The maximum erase voltage V2max at the cathode 21 of thesecond resistive switching element 20 is given asV2max=Vbe−Vce≅0.4V.

The maximum erase current during the erase operation is thus given by

${{{Max}.\;{Erase}}\mspace{14mu}{Current}} = {{Ipr}*{\frac{\beta}{\beta + 1}.}}$

FIG. 3C illustrates the operation of the memory cell including parasiticeffects in accordance with an embodiment of the present invention.

As described above with respect to FIG. 2, parasitic vertical bipolartransistors, for example, a first parasitic transistor 51 and a secondparasitic transistor 52 (FIG. 2B) are also formed along with the lateralbipolar transistor 50. However, the programming, erase, and readoperations of the differential resistive memory cell are similar to thatdescribed in FIG. 3B.

Similar to FIG. 3B, the first emitter/collector region 121 of thebipolar transistor 50 is initially at about 0V and the base current(I_(b)) is negligible (leakage current). The write voltage of the bitline may be coupled through a high resistance state R1. Thus, thevoltage at the anode 12 of the first resistive switching element 10(Vbl) increases after the application of the bit line voltage. At thebeginning of the programing operation, the collector current Ic_(a)through the bipolar transistor 50 and the collector current Ic_(p)through the first parasitic transistor 51 are negligible. The voltagepotential at the cathode 21 of the second resistive element 20 is pulleddown to about negative write voltage −V_(WRITE) on the application ofthe negative write voltage on the other first bit line BL1#. Further,the second parasitic transistor 52 is in an OFF state because theparasitic diode between the second emitter/collector region 122 and thebase of the second parasitic transistor 52 is under reverse bias.

During programming of the first resistive switching element 10, thevoltage of the anode 12 is given as

${Vbl} = {{( \frac{R\; 1}{{R\; 1} + {Rl}} )*( {{+ {Vwrite}} - {Vbe}} )} + {{Vbe}.}}$

The program current (IPR) is the same as previously describedirrespective of the parasitic transistors. The program current (IPR) isthe same as the emitter current Ie and is given as

${I\; e} = {\frac{{Vwrite} - {Vbe}}{{R\; 1} + {Rl}} = {{Ipr}.}}$

The collector current Ica through the bipolar transistor 50 is reducedas a part of the emitter current Ie flows through the first parasitictransistor 51. Thus, the collector current Ica is given by

${Ica} = {( \frac{\beta\; a}{{\beta\; a} + 1} ){( \frac{{Vwrite} - {Vbe}}{{R\; 1} + {R\; l}} ).}}$

The collector current Icp through the first parasitic transistor 51 isgiven by

${Icp} = {( \frac{\beta\; p}{{\beta\; p} + 1} ){( \frac{{Vwrite} - {Vbe}}{{R\; 1} + {R\; l}} ).}}$

In the above equations, βa is the common emitter current gain of thebipolar transistor 50 while βp is the common emitter gain of the firstparasitic transistor 51.

As described previously, the maximum erase current through the secondresistive element 20 is the collector current Ica flowing through thebipolar transistor 50 and is given by

$\frac{{V\; 2} + {Vwrite}}{R\; 2} = {{Ica} = {{{Max}.\mspace{11mu}{Erase}}\mspace{14mu}{{Current}.}}}$

The maximum erase current flowing through the second resistive switchingelement 20 is reduced because of the current flowing through the firstparasitic transistor 51. Therefore, the maximum erase current may bewritten as

${{Ipr}*\{ {( \frac{\beta\; a}{{\beta\; a} + 1} ) - ( \frac{\beta\; p}{{\beta\; p} + 1} )} \}} = {{{Max}.\mspace{11mu}{Erase}}\mspace{14mu}{{Current}.}}$

As a consequence, the operation of the memory cell is similar to thatdescribed above with respect to FIG. 3B although the current flowingthrough the second resistive switching element 20 is reduced because ofthe parasitic transistor. In both cases, the erase current through thesecond resistive switching element 20 is less than the program currentthrough the first resistive switching element 10. Because of theparasitic effects, the erase current is further reduced. However, thisreduction in erase current is controlled by βp, which in part iscontrolled by the base width of the parasitic transistor. Increasing thejunction depth of 110 accomplishes this. Other methods, known to thosewith ordinary skill in the art, could also reduce the current gain ofthe parasitic transistor. The resulting maximum erase current should besufficient to properly erase the memory cell.

FIG. 3D illustrates the operational states of the memory cell inaccordance with embodiments of the present invention.

Referring to FIG. 3-D, the differential memory cell has a first state(“I”) and a second state (“II”). The first state may be reached byprogramming the first resistive switching element 10 to a low resistancestate (LO) while erasing the second resistive switching element 20 to ahigh resistance state (HI). As described above in various embodiments,this first state may be achieved by applying a positive write voltage+V_(WRITE) on the other first bit line BL1 while applying a negativewrite voltage −V_(WRITE) on the first bit line BL1#, and applying anintermediate voltage (e.g., 0V) on the first word line WL1.

For illustration, the high resistance state is assumed to have aresistivity of 100 kΩ and the low resistance state is assumed to have aresistivity of 10 kΩ.

Next, the second state may be programmed by applying a positive writevoltage +V_(WRITE) on the other first bit line BL1# while applying anegative write voltage −V_(WRITE) on the first bit line BL1, andapplying an intermediate voltage (e.g., 0V) on the first word line WL1.

In the second state, the first resistive switching element 10 has a highresistance state (HI) while the second resistive switching element 20has a low resistance state (LO).

In a conventional memory cell,due to the design of the memory cell, thedifference between the high resistance state and the low resistancestate may be relatively small, which can result in read errors.

In contrast, in a differential memory cell described in variousembodiments, the difference between the first state and a second stateis magnified. Therefore, even if the high resistance state drifts to alower resistance and/or if the low resistance state drifts to a higherresistance, the memory cell's functionality is not hindered.

FIG. 3E illustrates a memory cell of the memory cell array during a readoperation in accordance with an embodiment of the present invention.

In one or more embodiments, suitable read voltages may be applied forreading the memory state of the differential memory cell. In oneembodiment, the different memory cell may be read by applying a samevoltage, e.g., a positive read voltage +VREAD, at both the first bitline pairs BL1 and BL1# and applying a lower voltage (e.g., 0V or−V_(READ)) at the base region of the bipolar transistor 50. Thedifference in the currents flowing in the first bit line pairs BL1 andBL1# yields the memory state of the differential memory cell. As duringprogramming, the fourth memory cell is inhibited by applying a positiveread voltage +V_(READ) on the remaining word lines (e.g., on the secondword line WL2). Measuring the currents through the first bit line pairsBL1 and BL1# and the first word line WL1 provides the resistance of thefirst and the second resistive switching elements 10 and 20.

In one embodiment, a B/L select transistor 310 is activated by a selectline. The device size of the B/L select transistor 310 is selected toobtain an acceptable ON resistance. The resistance Rread is selected incombination with the read voltage Vread for the difference between thevoltages V3 and V4 (|V3−V4|) to achieve practical data sensing for thedesired ranges of high resistance state (HI) and the low resistancestate (LO).

As described above, the read voltage Vread is greater than Vbe, thevoltage at the first emitter collector region 121 of the bipolartransistor 50. The third voltage V3 at the first input node of the senseamplifier 320 is given by the following equation.

${V\; 3} = {( {{Vread} - {Vbe}} )*{\frac{R\; 1}{{R\; 1} + {Rread}}.}}$

The fourth voltage V4 at the second input node of the sense amplifier320 is given by the following equation.

${V\; 4} = {( {{Vread} - {Vbe}} )*{\frac{R\; 2}{{R\; 2} + {Rread}}.}}$

Here, Vbe may be about 0.5V for the low resistance state (LO) and about0V for the high resistance state (HI).

FIG. 4, which includes FIGS. 4A-4E, illustrates a memory cell array inaccordance with an alternative embodiment of the present invention.

Unlike the prior embodiment, in this embodiment, the bipolar transistoris a NPN transistor 55. Consequently, the first resistive switchingdevice 10 and the second resistive switching device 20 have to beinterchanged (in other words reversed in polarity).

Referring to FIG. 4A, the NPN transistor 55 includes a firstcollector/emitter region 421, a second collector/emitter region 422 anda base region. As illustrated, the cathode 11 of the first resistiveswitching element 10 is coupled to a first bit line BL1 while thecathode 21 of the second resistive switching element 10 is coupled tothe other first bit line BL1#. Similarly, the anode 12 of the firstresistive switching element 10 is coupled to a first collector/emitterregion 421 while the anode 22 of the second resistive switching element20 is coupled to a second collector/emitter region 422. The base of theNPN transistor 55 is coupled to a first word line WL1.

The memory cell may be programmed to a first state (LO on the firstresistive switching element 10 and HI on the second resistive switchingelement 20) by applying a negative write voltage −V_(WRITE) at a firstbit line BL1 and a positive write voltage V_(WRITE) at the other firstbit line BL1#, and an intermediate voltage at the first word line WL1.Similarly, the memory cell may be programmed to a second state byreversing the voltage between the first bit line pairs. The fourthmemory cell (and other memory cells in the column of the first bit linepairs) may be inhibited by applying a negative write voltage −V_(WRITE)on the remaining word lines such as the first word line WL1.

FIGS. 4B-4E illustrate a structural implementation of the memory cellillustrated in FIG. 4A in accordance with embodiments of the presentinvention. FIG. 4B illustrates a top view, FIGS. 4C and 4D illustratecross-sectional views while FIG. 4E illustrates a magnifiedcross-sectional view of the resistive switching element.

Referring to FIG. 4C, a well region 110 comprising plurality of wordlines and having a p-type doping are formed as described in priorembodiments. In this embodiment, the substrate 100 may have an n-typedoping in one embodiment.

Alternatively, the well region 110 comprising the plurality of wordlines may be formed within a double well structure. In other words, ann-type well region 111 may be formed over a substrate 100, which mayhave a p-type doping and subsequently the well region 110 comprising theplurality of word lines may be formed within the n-type well region 111.

The plurality of doped regions 120 are formed over the well region 110comprising the plurality of word lines. The plurality of doped regions120 includes the first collector/emitter region 421 and the secondcollector/emitter region 422.

As in the prior embodiment, a plurality of contacts 130 couple theplurality of doped regions 120 to the resistive switching layer. As inprior embodiments, a plurality of resistive switching elements 140 isformed over the plurality of contacts 130 and a plurality of bit linecontacts 150 are formed over the plurality of resistive switchingelements 140. However, unlike the prior embodiments, the layers of theplurality of resistive switching elements 140 are reversed in thisembodiment.

FIG. 4E illustrates an element such as the first resistive switchingelement 10 of the plurality of resistive switching elements 140 inaccordance with an embodiment of the invention. As illustrated, thelocation of the cathode electrode layer 15 and the anode electrode layer17 are exchanged such that the anode electrode layer 17 is formed as abottom electrode and the cathode electrode layer 15 is formed as a topelectrode.

FIG. 5 illustrates a memory cell array having a NFET and blocking diodesin accordance with an alternative embodiment of the present invention.

Unlike the prior embodiments, in this embodiment, a field effecttransistor is used instead of the bipolar transistor. Referring to FIG.5, a field effect transistor 500 (such as the first NFET N1) is coupledbetween the anode 12 of the first resistive switching element 10 andanode 22 of the second resistive switching element 20.

A first source/drain region of the field effect transistor 500 iscoupled to the anode 12 of the first resistive switching element 10 anda second source range region of the field effect transistor 500 iscoupled to the anode 22 of the second resistive switching element 20.The gate of the field effect transistor 500 is coupled to a write wordline such as the first write word line WL1.

During transitioning from one state to another state, the field effecttransistor 500 is operated in inversion such that a source to draincurrent flows through the field effect transistor 500. In the absence ofa gate bias on the field effect transistor 500, the field effecttransistor 500 cuts offs the program/erase/read operation although aleakage current may pass through the field effect transistor 500.

The field effect transistor 500 may be an n-channel field effecttransistor (NFET) or a p-channel field effect transistor (PFET) as willbe described in FIG. 6.

As described above in prior embodiments, the first resistive switchingelement 10 may be switched from a high resistance state to a lowresistance state while simultaneously the current flowing through thefield effect transistor 500 is used to switch the second resistiveswitching element 20 from a low resistance state to a high resistancestate and vice versa.

As further illustrated, each memory cell has a first diode D1 andanother first diode D1# coupled between the anode 12 of the firstresistive switching element 10 and the anode 22 of the second resistiveswitching element 20. Similarly, the second memory cell has the seconddiode pair D2 and D2#, the third memory cell has the third diode pair D3and D3#, and the fourth memory cell has the fourth diode pair D4 andD4#.

In this illustrated embodiment, the field effect transistor is a NFET,and therefore each memory cell has an associated NFET (N1, N2, N3, andN4).

As illustrated, the anode 12 of the first resistive switching element 10is coupled to the n-side of the first diode D1 (i.e., cathode of thediode) and the anode 22 of the second resistive switching element 20 isalso coupled to the n-side of the other first diode D1#. Thus, theinterconnect 510 between the cathode of the first diode D1 and the firstsource/drain region of the NFET N1 as well as between the cathode of theother first diode D1# and the second source/drain region of the NFET N1may be formed through the semiconductor region of the substrate.Similarly, the connection between the p-side of the first diode D1 andthe p-side of the other first diode D1# may be through a semiconductorregion, i.e., the p-side of the first diode D1 and D1# may share acommon p-type region.

FIG. 6 illustrates a further embodiment of the differential memory cellarray having a PFET and blocking diodes in accordance with an embodimentof the present invention.

Unlike the embodiment illustrated in FIG. 5, in this embodiment a PFETis used instead of a NFET. Therefore each memory cell has an associatedPFET (P1, P2, P3, and P4).

Similarly, the source/drain regions of the PFET are of the same type ofdoping as the p-side of the diodes D1 and D1#. Therefore, the connectionbetween the anode of the first diode D1 to the first source/drain regionof the PFET P1 and the connection between the anode of the other firstdiode D1# to the second source/drain region of the PFET P1 may be madewithin the semiconductor substrate.

FIG. 7, which includes FIGS. 7A-7C, illustrates a memory deviceimplementing embodiments of the invention.

Referring to FIG. 7A, the memory device comprises a memory cell array200 (e.g., as described in various embodiments previously), accesscircuits 210, and program/erase circuits 220. The memory cell array 200may comprise a plurality of memory cells (CELL1, CELL2, CELL3, CELL4) asdescribed previously in FIGS. 1-6. The access circuits 210 provideelectrical connections to the memory cell array 200 so that the memorycells may be programmed, erased, and read. The access circuits 210 maybe located on one or more sides of the memory cell array 200. Forexample, the access circuits 210 may be located on opposite sides suchthat the potential may be applied across the memory units. The accesscircuits 210 may comprise word line drivers, bit line drivers as anexample.

The program and erase circuits 220 may provide program and erase signals(e.g., P/E₁, P/E₂) to the access circuits 210, which applies them to thememory cell array 200. The peak program or erase voltage may be higherthan or lower than a supply voltage. The program and erase circuits mayinclude charge pump circuits for generating higher than supply voltages,or step down voltage regulators and the like generating lower thansupply voltages. The program and erase circuits may also receive one ormore of the program and erase signals from an external circuit in someembodiments. In some embodiments, the program and erase circuits maycomprise program circuits physically separate from the erase circuits.

Referring to FIG. 7B, the program and erase circuits 220 and the readcircuits 250 may be connected such that one of them may be assertedthrough the access circuits 210, which may access control to the bitlines.

FIG. 7C illustrates a further embodiment of the memory device. Thememory device includes the program and erase circuits 220 and memorycell array 200 as described in FIG. 7A and FIG. 7B. The access circuitsmay include a column decoder 230 and a row decoder 240. In response toan address data, the column and the row decoders 230 and 240 may selecta group of memory cells for reading, programming, erasing. Further, thememory device may comprise read circuits 250 separate from the programand erase circuits 220. The read circuits 250 may include current and/orvoltage sense amplifiers. The memory device may further include aregister 260 for storing read data values from the memory cell array 200or to store data to be written into the memory cell array 200. Invarious embodiments, the register 260 may input and output data inparallel (i.e., bytes, words, and others). In some embodiments, theregister 260 may be accessed by serial data paths.

Input/output (I/O) circuits 270 may receive address values and writedata values, and output read data values. The received address valuesmay be applied to column and row decoders 230 and 240 to select memorycells. Read data from the register 260 may be output over the I/Ocircuits 270. Similarly, write data on I/O circuits 270 may be stored inregisters 260. A command decoder 290 may receive command data, which maybe passed on to the control logic 280. The control logic 280 may providesignals to control various circuits of the memory device.

FIG. 8 illustrates a schematic block diagram of a system implementingembodiments of the invention.

As illustrated in FIG. 8, the system may comprise the memory device 400,a processor 410, and output device 420, an input device 430, andoptionally a peripheral device 450. The memory device 400 may be formedas described in FIG. 7 in one or more embodiments and may comprise aplurality of memory units.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. As an illustration, the embodiments described in FIGS. 1-6may be combined each other in various embodiments. It is thereforeintended that the appended claims encompass any such modifications orembodiments.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. For example,it will be readily understood by those skilled in the art that many ofthe features, functions, processes, and materials described herein maybe varied while remaining within the scope of the present invention.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A memory cell comprising: a first resistiveswitching element comprising a first high resistance state and a firstlow resistance state and having a first terminal and a second terminal;a second resistive switching element comprising a second high resistancestate and a second low resistance state and having a first terminal anda second terminal; and a transistor including a first terminal, a secondterminal, and a third terminal, the first terminal of the transistorcoupled to the first terminal of the first resistive switching element,the second terminal of the transistor coupled to the first terminal ofthe second resistive switching element, the third terminal of thetransistor coupled to a word line, wherein the first terminals of thefirst and the second resistive elements are anode or cathode terminals,wherein the second terminals of the first and the second resistiveelements are cathode or anode terminals, wherein the first and thesecond terminals of the transistor are both coupled to anode terminalsof the first and the second resistive switching elements or to cathodeterminals of the first and the second resistive switching elements. 2.The memory cell of claim 1, wherein the transistor is configured toshunt the first terminal of the first resistive switching element withthe first terminal of the second resistive switching element so as toprogram the first resistive switching element and erase the secondresistive switching element simultaneously.
 3. The memory cell of claim1, wherein the second terminal of the first resistive switching elementis coupled to a first bit line, wherein the second terminal of thesecond resistive switching element is coupled to a second bit line. 4.The memory cell of claim 1, wherein the first resistive switchingelement is configured to have a resistance state opposite to the secondresistive switching element.
 5. The memory cell of claim 1, wherein thetransistor is a bipolar transistor.
 6. The memory cell of claim 5,wherein the first terminal of the transistor is a firstemitter/collector, wherein the second terminal of the transistor is asecond emitter/collector, and wherein the third terminal of thetransistor is a base.
 7. A memory cell comprising: a first resistiveswitching element having a first terminal and a second terminal; asecond resistive switching element having a first terminal and a secondterminal; and a three terminal transistor having a first terminal, asecond terminal, and a third terminal, the first terminal of the threeterminal transistor coupled to the first terminal of the first resistiveswitching element, the second terminal of the three terminal transistorcoupled to the first terminal of the second resistive switching element,the third terminal of the three terminal transistor coupled to a wordline, wherein the transistor is a PNP transistor, wherein the firstterminal of the first resistive switching element is a cathode terminalof the first resistive switching element, and wherein the first terminalof the second resistive switching element is a cathode terminal of thesecond resistive switching element.
 8. The memory cell of claim 5,wherein the bipolar transistor is a NPN transistor, wherein the firstterminal of the first resistive switching element is an anode terminalof the first resistive switching element, and wherein the first terminalof the second resistive switching element is a anode terminal of thesecond resistive switching element.
 9. A memory cell comprising: a firstresistive switching element having a cathode terminal and an anodeterminal; a second resistive switching element having a cathode terminaland an anode terminal; and a bipolar transistor having a firstemitter/collector, a second emitter/collector, and a base, the firstemitter/collector coupled to the cathode terminal of the first resistiveswitching element, the second emitter/collector coupled to the cathodeterminal of the second resistive switching element, the base coupled toa word line, the anode terminal of the first resistive switching elementcoupled to a first bit line of a bit line pair, the anode terminal ofthe second resistive switching element coupled to a second bit line ofthe bit line pair, the memory cell configured to store a first or asecond memory state.
 10. The memory cell of claim 9, wherein the bipolartransistor is a PNP transistor.
 11. The memory cell of claim 9, whereinthe bipolar transistor is a NPN transistor.
 12. The memory cell of claim9, wherein the first resistive switching element is configured to have aresistance state opposite to the second resistive switching element. 13.The memory cell of claim1, wherein the transistor comprises a bipolartransistor or a field effect transistor, and wherein both the first andthe second terminals of the transistor comprise a source/drain terminalor a emitter/collector terminal.
 14. The memory cell of claim 1, whereinthe first and the second terminals of the transistor are both coupled toanode terminals of the first and the second resistive switchingelements.
 15. The memory cell of claim 1, wherein the transistor is athree terminal device.
 16. The memory cell of claim 1, wherein the firstresistive switching element and the second resistive switching elementare configured to be in opposite resistive states.
 17. The memory cellof claim 1, wherein the memory cell has a first memory state when thefirst resistive switching element has the first high resistance stateand the second resistive switching element has the second low resistancestate, wherein the memory cell has a second memory state when the firstresistive switching element has the first low resistance state and thesecond resistive switching element has the second high resistance state.18. The memory cell of claim 1, wherein the transistor comprise: a firstdoped region comprising a first doping type disposed in a well regionhaving an opposite second doping type; and a second doping regioncomprising the first doping type disposed in the well region, the seconddoped region being separated from the first doped region by a portion ofthe well region, wherein the first doped region, the second dopedregion, and the portion of the well region form part of the transistor,wherein the first resistive switching element is disposed over andcoupled to the first doped region, wherein the second resistiveswitching element is disposed over and coupled to the second dopedregion, and wherein the word line is disposed in the well region.
 19. Amethod of operating a memory cell, the method comprising: providing amemory cell comprising: a first resistive switching element comprising afirst high resistance state and a first low resistance state and havinga first terminal and a second terminal; a second resistive switchingelement comprising a second high resistance state and a second lowresistance state and having a first terminal and a second terminal; anda transistor including a first terminal, a second terminal, and a thirdterminal, the first terminal of the transistor coupled to the firstterminal of the first resistive switching element, the second terminalof the transistor coupled to the first terminal of the second resistiveswitching element, the third terminal of the transistor coupled to aword line, wherein the first terminals of the first and the secondresistive elements are anode or cathode terminals, wherein the secondterminals of the first and the second resistive elements are cathode oranode terminals, wherein the first and the second terminals of thetransistor are both coupled to anode terminals of the first and thesecond resistive switching elements or to cathode terminals of the firstand the second resistive switching elements, wherein the second terminalof the first resistive switching element is coupled to a first bit line,and wherein the second terminal of the second resistive switchingelement is coupled to a second bit line; applying a positive writevoltage at the first bit line and a negative write voltage at the secondbit line; and activating the word line to an intermediate voltagebetween the positive write voltage and the negative write voltage so asto program the first resistive switching element and erase the secondresistive switching element simultaneously.
 20. The method of claim 19,wherein the transistor comprises a bipolar transistor or a field effecttransistor, and wherein both the first and the second terminals of thetransistor comprise a source/drain terminal or a emitter/collectorterminal.
 21. The method of claim 19, wherein the first and the secondterminals of the transistor are both coupled to anode terminals of thefirst and the second resistive switching elements.
 22. The method ofclaim 19, wherein, during a read operation, no current flows from thefirst bit line to the second bit line.
 23. The method of claim 22,wherein the read operation comprises: applying a read voltage at thefirst bit line and the second bit line, and applying an intermediatevoltage at the at the word line, wherein a memory state of the memorycell is identified by the difference in the current in the first bitline to the current in the second bit line.
 24. A memory cellcomprising: a well region having a first doping type disposed in asemiconductor substrate; isolation regions disposed in the semiconductorsubstrate and dividing the well region into a plurality of word lines; afirst doped region comprising an opposite second doping type disposed ina word line of the plurality of word lines; a second doped regioncomprising the second doping type disposed in the word line, the seconddoped region being separated from the first doped region by a portion ofthe well region, wherein the first doped region, the second dopedregion, and the portion of the well region form part of a lateralbipolar transistor; a first resistive switching element having a firsthigh resistance state and a first low resistance state and comprising aanode/cathode terminal and a corresponding cathode/anode terminal,wherein the anode/cathode terminal of the first resistive switchingelement is disposed over and coupled to the first doped region; and asecond resistive switching element having a second high resistance stateand a second low resistance state and comprising a anode/cathodeterminal and a corresponding cathode/anode terminal, wherein theanode/cathode terminal of the second resistive switching element isdisposed over and coupled to the second doped region.
 25. The memorycell of claim 24, further comprising: a first bit line disposed over thefirst resistive switching element, the first bit line coupled to thecathode/anode terminal of the first resistive switching element; and asecond bit line disposed over the second resistive switching element,the second bit line coupled to the cathode/anode terminal of the secondresistive switching element.
 26. The memory cell of claim 24, whereinthe anode/cathode terminal of the first and the second resistiveswitching elements are anode terminals so that the first and the seconddoped region of the transistor are both coupled to anode terminals ofthe first and the second resistive switching elements.
 27. The memorycell of claim 24, wherein the anode/cathode terminal of the first andthe second resistive switching elements are cathode terminals so thatthe first and the second doped regions of the transistor are bothcoupled to cathode terminals of the first and the second resistiveswitching elements.